Agricultural Bank of China (AgBank) said on Sunday it had fully exercised an over-allotment option for the Shanghai portion of its initial public offering, taking total proceeds from the offer to $22.1 billion, making the IPO the world's biggest.

The statement by AgBank, posted on the website of the Shanghai Stock Exchange, confirmed a Reuters report on Friday.

The exercise of the over-allotment, also known as a greenshoe, pushed AgBank's Hong Kong and Shanghai offering past Industrial and Commercial Bank of China's (ICBC) $21.9 billion IPO four years ago.

Over-allotments are released when demand for the shares in the after-market is heavy. Underwriters release the shares, set aside at the original IPO price, to the allocated holders who then become public stockholders.

AgBank had already exercised a similar option for its Hong Kong portion last month.

The exercise of the over-allotment brings the number of shares sold in AgBank's Hong Kong and Shanghai offerings to 54.79 billion, increasing the original $19.3 billion raised by 15 percent.

($1=6.77 Yuan)

(Reporting by Jason Subler and Samuel Shen; Editing by Erica Billingham)